DE4432859A1 - Strong flexible symmetrical aromatic polyamide micro-filtration membrane prodn. - Google Patents

Abstract

Prodn. of a symmetrical microfiltration membrane based on aromatic polyamide (I) involves dissolving (I) in solvent(s) in the presence of LiCl, casting the soln. on a smooth substrate to form a membrane, contacting this with humid air until it become turbid and then completing consolidation by immersion in water. The novel features are that: (a) the soln. is treated with polyvinyl-pyrrolidone (PVP) with a mol. wt. of over 50000 and with ethylene glycol or glycerol as non-solvent (II) for (I); and (b) the cell structure and pore size of the membrane are regulated by the dwell in humid air. The membrane is claimed per se. The membrane has cells with a dia. of 1-10, pref. 3-7, microns, and has an elongation at break of 30-50% in the moist state. The casting soln. contains DMF, DMA and/or NMP, esp. a mixt. of DMA/DMF or DMA/NMP, as solvent, up to 25, esp. 8-20 wt.% (I), 0.2-5 wt.% PVP, LiCl in an amt. of 5-100% w.r.t. (I) and 5-25, esp. 10-15 wt.% (II). It has a viscosity of 15-30 Pa.s. The membrane is contacted with air having at 80-100% RH and 15-30, esp. 20-28, more esp. 20 deg C for 5-15 min. The soln. is cast in a thickness of 50-300, esp. 80-150, more esp. 100, microns.

Description

The invention relates to microfiltration membranes based on aromatic polyamide and a process for their preparation according to Preamble of claim 1.

There are quite a few for the production of microfiltration membranes suitable polymers in use. A main area of application for such Membranes lie in the filtration of aqueous media, which means that such Membranes hydrophilic, i. H. must be spontaneously wettable with water in order to to get good filtration properties and these membranes on their To be able to test suitability in situ. The hydrophilicity of such membranes must also be obtained after multiple sterilization by flowing superheated steam stay. Many polymers differ based on this criterion lack of hydrophilicity, such as polypropylene or polysulfone. These polymers are only more or less complex Modification processes to get sufficiently hydrophilic. Other polymers, which are inherently hydrophilic, for example aliphatic polyamides (PA 66, PA 6, PA 46) have the disadvantage that under the conditions of Steam sterilization in the presence of even slight traces of oxygen be attacked and become brittle. Membranes made of polyester or Cellulose acetate already shows itself against the attack of weak acids or bases as unstable.

On the other hand, membranes are made of aromatic polyamide for their high Hydrophilicity and its good chemical resistance are known. A disadvantage microfiltration membranes made therefrom has so far been such that they do not  without the use of plasticizers for use in filter cartridges could be pleated because the membranes are so brittle when dry are that they break when they fold.

The invention was therefore based on the object of a membrane to provide aromatic polyamide, which without subsequent Addition of a plasticizer, such as glycerin, and without With the help of carrier materials or reinforcing materials high strength and flexibility that it turns out to be relatively thin Pleat the membrane well and process it into filter candles.

In terms of the method, the object is achieved with the features of patent claim 1 solved. Advantageous embodiments are the subject of the dependent claims. The membrane according to the invention is the subject of claim 13.

The basic process steps and the composition of the Casting solutions, with the exception of adding high molecular weight Polyvinyl pyrrolidones (PVP) are already known in the prior art, for example from DE-OS 40 02 386. PVP is an aid to Manufacture of polysulfone and polyethersulfone membranes also known (DE-OS 38 02 030 A1). In this polymer, however, they are used to to improve the hydrophilicity of the membrane. In the manufacture of The use of PVP is ultrafiltration membranes made from aromatic polyamide also known (DE 39 03 098 A1). Because in this document after the Phase inversion process is felled, however, will be asymmetrical Membranes formed, the stability of which is mediated by the skin layer.

The process provides that a casting solution with polyamide, Solvent, non-solvent, PVP and lithium chloride is formed. Important is that the PVP has a high average molecular weight (<50,000) (Number average). Preferably serve as solvent  Dimethylformamide (DMF), dimethylacetamide (DMAc) and N- Methyl pyrrolidone (NMP) or any mixture thereof, preferably but a mixture of dimethylacetamide and dimethylformamide or Dimethylacetamide and N-methylpyrrolidone. The PVP will be the solution in Proportions of preferably 0.2 to 5 percent by weight are added. Because in the present application, unlike in DE 39 03 098 A1, where Asymmetric ultrafiltration membranes are obtained, a symmetrical Microfiltration membrane is to be formed, must also an Non-solvent for the aromatic polyamide, either ethylene glycol or Glycerin, in proportions of preferably 5 to 25 percent by weight will. The proportion of lithium chloride in the casting solution is preferably 5 to 100% of the polyamide content, the polyamide content up to 25 percent by weight, preferably 8 to 20 percent by weight can.

The casting solution formed, which preferably has a viscosity of 15 to 30 Pas should then be placed on a base, for example a glass or metal plate, applied with the help of a doctor knife and to a Film thickness of about 50 to 300 microns extended. This film layer is then in a first stage over several minutes, preferably 5 to 15 minutes, with air of a relative humidity of preferably 80 to 100 percent 15-30 ° C, preferably 20 ° C brought into contact. After the clouding of the Membrane, the plate is then very quickly placed in a water bath (approx. 20 ° C) submerged. During this second stage, the membrane structure becomes final formed and solidified. The membrane properties of the membrane thus obtained can be influenced in large areas by the dwell time in moist air will. This shows a difference to that according to the previous status of Technology manufactured membranes, where the pore size of the membrane over the Mixing ratio of two solvents, e.g. B. DMF and DMAc controlled has been.  

Another significant difference to the membranes in the prior art Technology is that a far more fine-celled foam structure of the Membrane was generated. In the present application, between Membrane cells are the more or less spherical cavities in the Inside of the membrane, which is separated by thin walls from the Polymer material is separated from the neighboring cells, and pores distinguished. Since microfiltration membranes are each one open-cell foam structure, these cells are also among themselves connected. The junctions, which holes in the polymer walls between the cells are in the present application Pores.

In the membranes of the invention it surprisingly shows that by adding high molecular weight PVP the mechanical properties of aromatic polyamide membranes, especially elongation at break and folding resistance, could be greatly improved. This improvement The membrane properties are also surprising in that as well intensive washing of the membranes and after multiple steam sterilization the good mechanical properties are retained, although this means that most of the PVP is extracted. The membranes also break open Extraction or steam sterilization no longer when folding. While for Prior art membranes glycerin as plasticizer used in a post-treatment step for this purpose can at the membranes according to the invention can be dispensed with. Glycerin can but, as described above, in the manufacture of the membrane as a non-solvent be used for the aromatic polyamide.

The method according to the invention and the membranes obtained therewith are explained in more detail below using the examples and the illustrations.

Example 1 (membrane according to the invention)

By dissolving 10 parts by weight of aromatic polyamide (DuPont Nomex® fibers, type 450) and 3.5 parts by weight of lithium chloride in 19.5 Parts by weight of dimethylacetamide (DMAc) and 29.2 parts by weight Dimethylformamide (DMF) was made into a polymer concentrate. It was produced a second solution, which 2 parts by weight of PVP (MG 360 000), 8.3 parts by weight of DMAc, 12.5 parts by weight of DMF and 15 parts by weight Contained ethylene glycol.

The solutions were prepared at a temperature of 80 ° C. The mixture was stirred until clear solutions were obtained. After that the two solutions were allowed to cool and at a ratio of 53.2 Parts by weight of the first solution to 37.8 parts by weight of the second solution mixed together until a clear solution was also obtained again. It turned out to be useful to present the first solution and then the second solution slowly over a period of about two hours to add precipitates of poorly soluble polymer components avoid. The finished polymer solution was then brought to room temperature let cool. The solution remained clear and could now Membrane production can be used. This casting solution then showed the following Composition on: aromatic polyamide 9.4% by weight, solvent 68.64% by weight, PVP 2.2% by weight, ethylene glycol 16.48% by weight and LiCl 3.24% by weight.

With this solution, a series of tests was carried out by Part of this casting solution with a doctor knife on a glass plate into one Film with a thickness of 200 µm was spread out. With the help of an air conditioner Was air with 95% relative humidity at a Temperature of 25 ° C at a speed of 0.5 m / min. blown. The turbidity of the polymer solution was observed and after one certain time the glass plate with the polymer solution in water of 20 ° C immersed. The results are shown in Table 1 below.  

Table 1

The membrane thickness of the finished membrane was 100 µm. SEM images of the membrane cross-section were taken from the membrane marked with an asterisk and are shown in FIGS . 1a-d.

Example 2

Analogously to Example 1, a solution of 9% by weight of aromatic polyamide was 2.7% by weight of LiCl, 64.3% by weight of DMAc, 22% by weight of ethylene glycol and 2 % By weight of PVP (MW 360,000).

A test series was carried out with this solution, in each of which part of this solution with a doctor knife on a glass plate into one Film with a thickness of 200 µm was spread out. With the help of an air conditioner Was air with 95% relative humidity at a Temperature of 25 ° C at a speed of 0.5 m / min. blown. The turbidity of the polymer solution was observed and after one  certain time the glass plate with the respective polymer solution in water immersed at 20 ° C. The results are shown in Table 2.

Table 2

Example 3 (comparative example)

According to Example 3 of DE 39 03 098 A1, a solution of 7% by weight aromatic polyamide (Nomex Trp 450), 3.1% by weight PVP K 30 and 3.2 % By weight calcium chloride in 86.7% by weight N-methylpyrrolidone. The Polymer solution was added to a glass plate using a doctor knife poured a film of 200 microns thick. By immediately immersing the coated glass plate in water at 14 ° C, the polymer was coagulated. The resulting membrane had an exceptionally weak mechanical Strength and could therefore not be investigated further.  

Example 4 (comparative example)

The same polymer solution as in the previously described example was prepared, but using dimethylacetamide instead of N-methylpyrrolidone as solvent. The resulting membrane was washed out in water. The membrane was mechanically somewhat more stable than that described in the previous example. Scanning electron micrographs of the membrane obtained were produced, which are shown in FIGS. 2a, b. From the photographs it can be seen that it is an asymmetrical membrane with a skin layer on one side and with finger-like cavities in the membrane structure. This membrane is therefore not suitable for microfiltration, but can only be used for ultrafiltration purposes.

Example 5 (comparative example)

A membrane according to Example 1 of DE 40 02 386 was produced. The membrane obtained had a blowing point of 2.7 bar and a water flow of 35.2 ml / cm 2 min bar. The membrane had a thickness of 95 µm. SEM images of the membrane cross section were taken. These are shown in FIGS . 3a, b.

The mechanical properties were the same as that of the membrane of the example 1 (membrane according to the invention) compared. The parameters were the Bursting pressure, tensile strength, elongation at break and tear resistance certainly.

Determination of tensile strength and elongation at break

The tests were carried out in accordance with DIN 53 455. The Membrane samples consisted of strips 14 cm long and 3 cm wide. The Membrane samples were approx. 100 µm thick. In preliminary tests it was found that the mechanical properties of the membranes in Transverse and longitudinal direction in relation to the manufacturing process none showed significant differences, ie the membranes are isotopes Showed strength behavior.

One was used to measure tensile strength and elongation at break Membrane strips clamped in a measuring apparatus so that the length of the measuring membrane was 10 cm. The measuring speed was 1 mm / sec. The force and the elongation were determined at which the membrane tore.

Determination of tear resistance

The tear strength was determined in terms of tear strength and elongation at break measured according to the previously described procedure. Before was the membrane strip to be measured in the middle of the test piece Cut with scissors 5 mm at right angles to the edge.

Determination of the burst pressure

Circular discs with a diameter of 50 mm were made from the membranes punched out. These membrane discs were each in a membrane holder inserted, and covered with a metal washer, a hole in the middle with a diameter of 10 mm. The membrane was from the back pressurized with a pressure increase of 1 bar / min.

Determination of the folding test

A circular sample of 50 mm was made from each of the membranes Punched out diameter. This membrane sample became symmetrical once folded and between two square glass plates with 10 cm edge length placed. The top glass plate was weighing for one minute 10 kg loaded. The samples were then identified on the light microscope Checked for damage.

Because of the mechanical properties of the polyamide membrane Plasticizers such as water or glycerin can be greatly influenced, the samples were water wet (wet) and dry (dried and 2 hours conditioned at 50% relative humidity at 20 ° C) and with and without Aftertreatment with glycerin (10% based on polymer content) tested. In the following Table 3 are for the burst pressure, the tensile strength, the Elongation at break and tear strength are the mean values from 5 measurements juxtaposed.

Table 3

From the table it can be seen that the elongation at break and thus the Deformability of the membrane according to Example 1 in the wet state is essential is larger than in the comparison membrane produced according to the example above. In the folding test, the sample according to the comparative example was without Glycerin impregnation totally broken while using the sample Glycerin impregnation (10%) showed fine cracks on the outer edge. In contrast, both samples according to Example 1 were completely in order.

Comparison of the membrane pore structures according to Example 1 and Example 5 based on SEM images:

Both membrane samples were frozen with liquid nitrogen and then broken in the cold. This ensured that the pore structure of the membrane samples was largely unadulterated. The Samples were sputtered with gold, as is known in the art and then examined in a scanning electron microscope. In the figures reproduced images of the membrane cross-sections.

It shows:

Figure 1a shows the cross section of the membrane according to the invention according to Example 1 in 350 times magnification.

FIG. 1b shows the cross section of the membrane of the invention according to Example 1 in 1000 times magnification;

FIG. 1c is the cross section of the membrane of the invention according to Example 1 in 5000 times magnification;

Fig. 1d the cross section of the membrane of the invention according to Example 1 in 10 000 times magnification;

Fig. 2a shows the cross section of the membrane according to the prior art according to Example 4 × 350 in magnification;

FIG. 2b shows the cross section of the membrane according to the prior art according to Example 4 times in 1000 magnification;

FIG. 3a shows the cross section of the membrane according to the prior art according to Example 5 × 350 in magnification;

Fig. 3b shows the cross section of the membrane according to the prior art according to Example 5 in 1000 times magnification.

In FIGS. 2a and 2b, one can observe typical of asymmetric membranes cavernous structures below the skin layer. Such structures are not suitable for the task of the membranes according to the invention.

If one compares the image of FIGS . 1b and 3b with one another, it can be seen that the cell structure of the membrane according to Example 5 can be clearly seen even at a magnification of 1000 times. The membrane cells in FIGS . 3a and 3b have a diameter of approximately 2 to 3 μm. It should be noted that this is a membrane with a nominal pore size of approx. 0.2 µm. The actual pores that determine the separation behavior of the membrane are therefore significantly smaller than the cells.

In the membrane according to the invention according to FIG. 1b, the individual cells are not yet recognizable at a magnification of 1000 times. Only at 5000-fold magnification according to FIG. 1c can cells be recognized which in the illustration have approximately the same size as the cells according to comparative example 5 ( FIG. 3b) at 1000-fold magnification. The cells of the membrane according to the invention are thus approximately 5 times smaller than the cells of the membrane produced according to Comparative Example 5. The cells of the membrane according to the invention have a diameter of approximately 0.5 μm.

In addition, the structure of the two membranes is according to Examples 1 and 5 very different. While the cells of the according to Comparative Example 5 membrane produced are approximately spherical and partially have closed walls, the membrane of the invention shows a three-dimensional network structure in which the pores are approximately the same Have diameters like the cells themselves.

The structural differences therefore have a significant influence on the Properties of the two membrane types. Since membrane pores in the after Membranes manufactured in the phase inversion process are never all exactly the same size are, but always have a pore size distribution, is next to the Pore size distribution the number of pore passages over the Membrane cross section crucial for that Retention characteristics of the membranes. The smaller the cell size of one Membrane with the same nominal pore size, the more pores a must Particles pass to get through the membrane. So that increases but even then the likelihood that this particle will be retained becomes. For these reasons, the one made according to Example 1 In principle, the membrane according to the invention has better retention characteristics than the membrane produced according to Comparative Example 5.

The differences in the mechanical behavior of the two membrane types can can also be derived from the different membrane structure. As above already stated, the membrane according to experimental example 5 has a structure with relatively large cells and large membrane walls between them Cells on. Since aromatic polyamide is generally a relatively rigid material is therefore, in principle, this membrane structure tends to be more mechanical To break stress such as stretching, compression or folding. In contrast  the membrane according to the invention according to Example 1 has a more compact one Structure based on mechanical stress on the cell walls attacking forces distributed much better across the entire network structure can be compared to the relatively large walls of the membrane Experimental example 5. This membrane can be deformed considerably more without stressing the individual membrane walls to their breaking point were. This explained the overall better elongation at break of the membrane according to the invention compared to membranes according to the prior art Technology.

Claims (16)

1. A process for producing a symmetrical microfiltration membrane based on aromatic polyamide, in which the aromatic polyamide is dissolved in a solvent or a solvent mixture in the presence of LiCl, the solution is then poured out on a smooth surface to form a membrane and with moist air until cloudy brought into contact and then finally solidified by immersion in a water bath, characterized in that polyvinylpyrrolidone with an average molecular weight <50,000 and a non-solvent for the polyamide from the group consisting of ethylene glycol and glycerol were added to the solution and the cell structure and pore size of the membrane were added to the solution Dwell time can be set in moist air. 2. The method according to claim 1, characterized in that the Casting solution is composed of a solvent, selected from the group dimethylformamide, dimethylacetamide and N- Methylpyrrolidone or a mixture thereof, an aromatic polyamide with a proportion of up to 25% by weight, preferably from 8 to 20 percent by weight, PVP in a proportion of 0.2 to 5 percent by weight, LiCl in a proportion of 5 to 100% of the polyamide proportion and one Proportion of 5 to 25 percent by weight of a non-solvent for the Polyamide. 3. Process according to claims 1 and 2, characterized in that the viscosity of the casting solution is 15 to 30 Pas.   4. The method according to claims 1 to 3, characterized in that contacting with air at 80 to 100% relative humidity in Temperature range of 15-30 ° C takes place. 5. The method according to claim 4, characterized in that the Contact with air in the temperature range of 20-28 ° C takes place. 6. The method according to claim 4 or 5, characterized in that the Contact with air at 20 ° C. 7. The method according to any one of claims 4 to 6, characterized in that that the air contact time is 5 to 15 minutes. 8. The method according to any one of claims 1 to 7, characterized in that as a solvent mixture dimethylacetamide / dimethylformamide or dimethylacetamide / N-methylpyrrolidone is used. 9. Method according to one of claims 1 to 8, characterized in that the proportion of the non-solvent in the casting solution 10 to 15 Weight percent. 10. The method according to any one of claims 1 to 9, characterized in that the casting solution on the base to a thickness of 50 to 300 µm is applied. 11. The method according to claim 10, characterized in that the Pouring solution on the base to a thickness of 80 to 150 µm is applied.   12. The method according to claims 10 or 11, characterized in that the casting solution on the base with a thickness of 100 microns is applied. 13. Microfiltration membrane made of aromatic polyamide, thereby characterized in that it contains an addition of PVP, is symmetrical and their cells have a diameter between 1 and 10 microns. 14. Microfiltration membrane according to claim 13, characterized in that the PVP has a number average molecular weight <50,000 having. 15. microfiltration membrane according to claim 13 and 14, characterized characterized in that the cells have a diameter between 3 and 7 microns exhibit. 16. Microfiltration membrane according to one of claims 13 to 15, characterized characterized that their elongation at break when wet 30-50% is.